1. Field of the Invention
The present invention relates generally to fuel cells and, more specifically, to measuring certain operating characteristics of such fuel cells, fuel cell stacks and fuel cell arrays.
2. Background Information
Fuel cells are devices in which an electrochemical reaction is used to generate electricity. A variety of materials may be suited for use as a fuel depending upon the materials chosen for the components of the cell. Organic materials, such as methanol or natural gas, are attractive fuel choices due to the their high specific energy.
Fuel cell systems may be divided into “reformer-based” systems (i.e., those in which the fuel is processed in some fashion to extract hydrogen from the fuel before it is introduced into the fuel cell system) or “direct oxidation” systems in which the fuel is fed directly into the cell without the need for separate internal or external processing. Most currently available fuel cells are reformer-based fuel cell systems. However, because fuel processing is expensive and generally requires expensive components, which occupy significant volume, reformer based systems are presently limited to comparatively large, high power applications.
Direct oxidation fuel cell systems may be better suited for a number of applications in smaller mobile devices (e.g., mobile phones, handheld and laptop computers), as well as in some larger applications. In direct oxidation fuel cells of interest here, a carbonaceous liquid fuel (typically methanol or an aqueous methanol solution) is introduced to the anode face of a membrane electrode assembly (MEA).
One example of a direct oxidation fuel cell system is a direct methanol fuel cell system or DMFC system. In a DMFC system, a mixture comprised predominantly of methanol, or methanol and water, is used as fuel (the “fuel mixture”), and oxygen, preferably from ambient air, is used as the oxidizing agent. The fundamental reactions are the anodic oxidation of the fuel mixture into CO2, protons, and electrons; and the cathodic combination of protons, electrons and oxygen into water. The overall reaction may be limited by the failure of either of these reactions to proceed to completion at an acceptable rate, as is discussed further hereinafter.
Typical DMFC systems include a fuel source, fluid and effluent management systems, and air management systems, as well as a direct methanol fuel cell (“fuel cell”). The fuel cell typically consists of a housing, hardware for current collection and fuel and air distribution, and a membrane electrode assembly (“MEA”) disposed within the housing.
The electricity generating reactions and the current collection in a direct oxidation fuel cell system generally take place within the MEA. In the fuel oxidation process at the anode, the products are protons, electrons and carbon dioxide. Protons (from hydrogen found in the fuel and water molecules involved in the anodic reaction) are separated from the electrons. The protons migrate through the membrane electrolyte, which is impermeable to the electrons. The electrons travel through an external circuit, which connects the load, and are united with the protons and oxygen molecules in the cathodic reaction, thus providing electrical power from the fuel cell.
A typical MEA includes a centrally disposed protonically-conductive, electronically non-conductive membrane (“PCM”, sometimes also referred to herein as “the catalyzed membrane”). One example of a commercially available PCM is NAFION® a registered trademark of E.I. Dupont de Nemours and Company, a cation exchange membrane based on polyperflourosulfonic acid, in a variety of thicknesses and equivalent weights. The PCM is typically coated on each face with an electrocatalyst such as platinum, or platinum/ruthenium mixtures or alloy particles. On either face of the catalyst coated PCM, the electrode assembly typically includes a diffusion layer. The diffusion layer on the anode side is employed to evenly distribute the liquid fuel mixture across the catalyzed anode face of the PCM, while allowing the gaseous product of the reaction, typically carbon dioxide, to move away from the anode face of the PCM. In the case of the cathode side, a diffusion layer is used to allow a sufficient supply of and a more uniform distribution of gaseous oxygen across the cathode face of the PCM, while minimizing or eliminating the collection of liquid, typically water, on the cathode aspect of the PCM. Each of the anode and cathode diffusion layers also assist in the collection and conduction of electric current from the catalyzed PCM through the load.
Direct oxidation fuel cell systems for portable electronic devices should be as small as possible at the power output required. The power output is governed by the rate of the reactions that occur at the anode and the cathode of the fuel cell. More specifically, the anode process in direct methanol fuel cells based on acidic electrolytes, including polyperflourosulfonic acid and similar polymer electrolytes, involves a reaction of one molecule of methanol with one molecule of water. In this process, the oxygen atom in the water molecule is electrochemically activated to complete the oxidation of methanol to a final CO2 product in a six-electron process.
More specifically, direct methanol fuel cell system produces electricity without combustion by oxidizing a carbonaceous fuel (typically methanol in an aqueous solution) on a catalyzed protonically conductive membrane.
The electrochemical reaction equations are as follows:Anode: CH3OH+H2O═CO2+6H++6e−  Equation 1Cathode: 6H++6e−+3/2O2=3H2O  Equation 2Net Process: CH3OH+3/2O2=CO2+2H2O  Equation 3
Generation of electricity continues until one of the fluids is not available. DMFCs are typically described as “on” i.e. providing electrical current by reacting the fuel and oxygen to generate water, or “off” i.e. at least one fluid is not available because all fuel has been consumed, or air (or other source of oxygen) is prevented from reaching the cathode face of the PCM. Those skilled in the art will recognize that fuel can be delivered to the anode aspect of the MEA as a liquid, or in vaporous form.
Thus, the efficiency of a direct methanol fuel cell system is dependent in part on the amount of fluids and products that are present in the active catalyzed membrane areas and also depends upon adequate hydration of the membrane. For example, particularly in a vapor fed cell, there is a tendency for the catalyzed membrane to dry out during operation, or when the fuel cell is shut down. This is because the vapor feed is not aqueous, instead it is substantially pure fuel, such that there is essentially no excess water on the anode side to keep the membrane hydrated. However, as stated, the membrane should remain well-hydrated for optimal performance. Further, when the fuel cell is not in operation, the membrane of a microfuel cell system may become dehydrated due, at least in part, to the lack of water generation at the cathode. This also undesirable particularly in situations when the fuel cell is in a standby status, such as when the fuel cell system is employed in a hybrid power source that contains the fuel cell and a battery. When the battery discharges, or during re-charge, the fuel cell begins to operate to power the device. However, while the battery (or other power source) is providing power to the device, the fuel cell may be in a shut down state in which the water-generating cathodic reactions do not occur. During this time period, cell dehydration can occur, and if it does, the fuel cell may then not be able to provide sufficient power when needed, but will require that the membrane electrolyte become rehydrated to allow for the proper reactions to occur.
Accordingly, it is desirable to determine whether the cell is adequately hydrated in order to facilitate the continued generation of electricity, and in order to avoid the time lag associated with rehydrating the membrane electrolyte and/or the fuel cell system. It has not always been straightforward, however, to obtain a reading or measurement of the amount of hydration existing at any one particular time in the membrane. Given the number of internal components in a microfuel cell, it is not always easy to measure the various operating characteristics of such fuel cells, particularly when the fuel cells are operated during elevated temperature runs.
It has been observed that fuel cell stack or array resistance is related to cell hydration. More specifically, cell resistance is a function of many things, including membrane material, geometry, compression, age and hydration. Of these, the one that changes most rapidly (in a matter of several minutes) is cell hydration. Thus, using a resistance measurement it is possible to determine whether the cell is adequately hydrated. More specifically, if resistance is increasing over time, this in an indication that the cell is drying out. If resistance is decreasing over time, this in an indication that the cell is becoming more hydrated and, perhaps, too hydrated.
Thus, the fuel cell stack (or array) resistance can be measured to evaluate and predict cell hydration. In practice, however, this type of measurement has required the use of an integrated fuel cell testing system such as that provided by Arbin Instruments of College Station, Texas. Such instrumentation is bulky, expensive and may require a separate power supply. Requiring additional hardware components is particularly disadvantageous in microfuel cell application where size and form factors are critical.
There remains a need therefore, for a more easily implemented method and apparatus for measuring series resistance in a direct oxidation fuel cell, a direct oxidation fuel cell stack or a direct oxidation fuel cell array.
It is thus an object of the invention to provide a method and apparatus for readily measuring the resistance of the fuel cell, the fuel cell stack or fuel cell array, which can in turn provide information about hydration within a fuel cell and more particularly, about adequate hydration of the membrane electrolyte.